carbon-lang/executable_semantics/interpreter/interpreter.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "executable_semantics/interpreter/interpreter.h"

#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <optional>
#include <utility>
#include <vector>

#include "common/check.h"
#include "executable_semantics/ast/expression.h"
#include "executable_semantics/ast/function_definition.h"
#include "executable_semantics/interpreter/stack.h"
#include "executable_semantics/interpreter/typecheck.h"
#include "executable_semantics/tracing_flag.h"

namespace Carbon {

State* state = nullptr;

auto PatternMatch(const Value* pat, const Value* val, Env,
                  std::list<std::string>*, int) -> std::optional<Env>;
auto Step() -> void;
auto GetMember(const Value* v, const std::string& f, int line_num) -> Address;
//
// Auxiliary Functions
//

auto Heap::AllocateValue(const Value* v) -> Address {
  // Putting the following two side effects together in this function
  // ensures that we don't do anything else in between, which is really bad!
  // Consider whether to include a copy of the input v in this function
  // or to leave it up to the caller.
  CHECK(v != nullptr);
  Address a = values_.size();
  values_.push_back(v);
  alive_.push_back(true);
  return a;
}

auto Heap::Read(Address a, int line_num) -> const Value* {
  this->CheckAlive(a, line_num);
  return values_[a];
}

auto Heap::Write(Address a, const Value* v, int line_num) -> void {
  CHECK(v != nullptr);
  this->CheckAlive(a, line_num);
  values_[a] = v;
}

void Heap::CheckAlive(Address address, int line_num) {
  if (!alive_[address]) {
    std::cerr << line_num << ": undefined behavior: access to dead value ";
    PrintValue(values_[address], std::cerr);
    std::cerr << std::endl;
    exit(-1);
  }
}

auto CopyVal(const Value* val, int line_num) -> const Value* {
  switch (val->tag()) {
    case ValKind::TupleValue: {
      std::vector<TupleElement> elements;
      for (const TupleElement& element : val->GetTupleValue().elements) {
        const Value* new_element =
            CopyVal(state->heap.Read(element.address, line_num), line_num);
        Address new_address = state->heap.AllocateValue(new_element);
        elements.push_back({.name = element.name, .address = new_address});
      }
      return Value::MakeTupleValue(std::move(elements));
    }
    case ValKind::AlternativeValue: {
      const Value* arg = CopyVal(
          state->heap.Read(val->GetAlternativeValue().argument, line_num),
          line_num);
      Address argument_address = state->heap.AllocateValue(arg);
      return Value::MakeAlternativeValue(val->GetAlternativeValue().alt_name,
                                         val->GetAlternativeValue().choice_name,
                                         argument_address);
    }
    case ValKind::StructValue: {
      const Value* inits = CopyVal(val->GetStructValue().inits, line_num);
      return Value::MakeStructValue(val->GetStructValue().type, inits);
    }
    case ValKind::IntValue:
      return Value::MakeIntValue(val->GetIntValue());
    case ValKind::BoolValue:
      return Value::MakeBoolValue(val->GetBoolValue());
    case ValKind::FunctionValue:
      return Value::MakeFunctionValue(val->GetFunctionValue().name,
                                      val->GetFunctionValue().param,
                                      val->GetFunctionValue().body);
    case ValKind::PointerValue:
      return Value::MakePointerValue(val->GetPointerValue());
    case ValKind::ContinuationValue:
      // Copying a continuation is "shallow".
      return val;
    case ValKind::FunctionType:
      return Value::MakeFunctionType(
          CopyVal(val->GetFunctionType().param, line_num),
          CopyVal(val->GetFunctionType().ret, line_num));

    case ValKind::PointerType:
      return Value::MakePointerType(
          CopyVal(val->GetPointerType().type, line_num));
    case ValKind::IntType:
      return Value::MakeIntType();
    case ValKind::BoolType:
      return Value::MakeBoolType();
    case ValKind::TypeType:
      return Value::MakeTypeType();
    case ValKind::AutoType:
      return Value::MakeAutoType();
    case ValKind::ContinuationType:
      return Value::MakeContinuationType();
    case ValKind::StructType:
    case ValKind::ChoiceType:
    case ValKind::BindingPlaceholderValue:
    case ValKind::AlternativeConstructorValue:
      return val;  // no need to copy these because they are immutable?
      // No, they need to be copied so they don't get killed. -Jeremy
  }
}

void Heap::DeallocateSubObjects(const Value* val) {
  switch (val->tag()) {
    case ValKind::AlternativeValue:
      Deallocate(val->GetAlternativeValue().argument);
      break;
    case ValKind::StructValue:
      DeallocateSubObjects(val->GetStructValue().inits);
      break;
    case ValKind::TupleValue:
      for (const TupleElement& element : val->GetTupleValue().elements) {
        Deallocate(element.address);
      }
      break;
    default:
      break;
  }
}

void Heap::Deallocate(Address address) {
  if (alive_[address]) {
    alive_[address] = false;
    DeallocateSubObjects(values_[address]);
  } else {
    std::cerr << "runtime error, deallocating an already dead value"
              << std::endl;
    exit(-1);
  }
}

void PrintEnv(Env values, std::ostream& out) {
  for (const auto& [name, address] : values) {
    out << name << ": ";
    state->heap.PrintAddress(address, out);
    out << ", ";
  }
}

//
// Frame and State Operations
//

void PrintFrame(Frame* frame, std::ostream& out) {
  out << frame->name;
  out << "{";
  Action::PrintList(frame->todo, out);
  out << "}";
}

void PrintStack(Stack<Frame*> ls, std::ostream& out) {
  if (!ls.IsEmpty()) {
    PrintFrame(ls.Pop(), out);
    if (!ls.IsEmpty()) {
      out << " :: ";
      PrintStack(ls, out);
    }
  }
}

void Heap::PrintHeap(std::ostream& out) {
  for (Address i = 0; i < values_.size(); ++i) {
    PrintAddress(i, out);
    out << ", ";
  }
}

auto Heap::PrintAddress(Address a, std::ostream& out) -> void {
  if (!alive_[a]) {
    out << "!!";
  }
  PrintValue(values_[a], out);
}

auto CurrentEnv(State* state) -> Env {
  Frame* frame = state->stack.Top();
  return frame->scopes.Top()->values;
}

void PrintState(std::ostream& out) {
  out << "{" << std::endl;
  out << "stack: ";
  PrintStack(state->stack, out);
  out << std::endl << "heap: ";
  state->heap.PrintHeap(out);
  if (!state->stack.IsEmpty() && !state->stack.Top()->scopes.IsEmpty()) {
    out << std::endl << "values: ";
    PrintEnv(CurrentEnv(state), out);
  }
  out << std::endl << "}" << std::endl;
}

//
// More Auxiliary Functions
//

auto ValToInt(const Value* v, int line_num) -> int {
  switch (v->tag()) {
    case ValKind::IntValue:
      return v->GetIntValue();
    default:
      std::cerr << line_num << ": runtime error: expected an integer"
                << std::endl;
      exit(-1);
  }
}

auto ValToBool(const Value* v, int line_num) -> int {
  switch (v->tag()) {
    case ValKind::BoolValue:
      return v->GetBoolValue();
    default:
      std::cerr << "runtime type error: expected a Boolean" << std::endl;
      exit(-1);
  }
}

auto ValToPtr(const Value* v, int line_num) -> Address {
  switch (v->tag()) {
    case ValKind::PointerValue:
      return v->GetPointerValue();
    default:
      std::cerr << "runtime type error: expected a pointer, not ";
      PrintValue(v, std::cerr);
      std::cerr << std::endl;
      exit(-1);
  }
}

// Returns *continuation represented as a list of frames.
//
// - Precondition: continuation->tag == ValKind::ContinuationV.
auto ContinuationToVector(const Value* continuation, int sourceLocation)
    -> std::vector<Frame*> {
  if (continuation->tag() == ValKind::ContinuationValue) {
    return continuation->GetContinuationValue().stack;
  } else {
    std::cerr << sourceLocation << ": runtime error: expected an integer"
              << std::endl;
    exit(-1);
  }
}

auto EvalPrim(Operator op, const std::vector<const Value*>& args, int line_num)
    -> const Value* {
  switch (op) {
    case Operator::Neg:
      return Value::MakeIntValue(-ValToInt(args[0], line_num));
    case Operator::Add:
      return Value::MakeIntValue(ValToInt(args[0], line_num) +
                                 ValToInt(args[1], line_num));
    case Operator::Sub:
      return Value::MakeIntValue(ValToInt(args[0], line_num) -
                                 ValToInt(args[1], line_num));
    case Operator::Mul:
      return Value::MakeIntValue(ValToInt(args[0], line_num) *
                                 ValToInt(args[1], line_num));
    case Operator::Not:
      return Value::MakeBoolValue(!ValToBool(args[0], line_num));
    case Operator::And:
      return Value::MakeBoolValue(ValToBool(args[0], line_num) &&
                                  ValToBool(args[1], line_num));
    case Operator::Or:
      return Value::MakeBoolValue(ValToBool(args[0], line_num) ||
                                  ValToBool(args[1], line_num));
    case Operator::Eq:
      return Value::MakeBoolValue(ValueEqual(args[0], args[1], line_num));
    case Operator::Ptr:
      return Value::MakePointerType(args[0]);
    case Operator::Deref:
      std::cerr << line_num << ": dereference not implemented yet\n";
      exit(-1);
  }
}

// Globally-defined entities, such as functions, structs, choices.
Env globals;

void InitGlobals(std::list<Declaration>* fs) {
  for (auto const& d : *fs) {
    d.InitGlobals(globals);
  }
}

auto ChoiceDeclaration::InitGlobals(Env& globals) const -> void {
  VarValues alts;
  for (const auto& [name, signature] : alternatives) {
    auto t = InterpExp(Env(), signature);
    alts.push_back(make_pair(name, t));
  }
  auto ct = Value::MakeChoiceType(name, std::move(alts));
  auto a = state->heap.AllocateValue(ct);
  globals.Set(name, a);
}

auto StructDeclaration::InitGlobals(Env& globals) const -> void {
  VarValues fields;
  VarValues methods;
  for (auto i = definition.members->begin(); i != definition.members->end();
       ++i) {
    switch ((*i)->tag) {
      case MemberKind::FieldMember: {
        auto t = InterpExp(Env(), (*i)->u.field.type);
        fields.push_back(make_pair(*(*i)->u.field.name, t));
        break;
      }
    }
  }
  auto st = Value::MakeStructType(*definition.name, std::move(fields),
                                  std::move(methods));
  auto a = state->heap.AllocateValue(st);
  globals.Set(*definition.name, a);
}

auto FunctionDeclaration::InitGlobals(Env& globals) const -> void {
  auto pt = InterpExp(globals, definition.param_pattern);
  auto f = Value::MakeFunctionValue(definition.name, pt, definition.body);
  Address a = state->heap.AllocateValue(f);
  globals.Set(definition.name, a);
}

// Adds an entry in `globals` mapping the variable's name to the
// result of evaluating the initializer.
auto VariableDeclaration::InitGlobals(Env& globals) const -> void {
  auto v = InterpExp(globals, initializer);
  Address a = state->heap.AllocateValue(v);
  globals.Set(name, a);
}

//    { S, H} -> { { C, E, F} :: S, H}
// where C is the body of the function,
//       E is the environment (functions + parameters + locals)
//       F is the function
void CallFunction(int line_num, std::vector<const Value*> operas,
                  State* state) {
  switch (operas[0]->tag()) {
    case ValKind::FunctionValue: {
      // Bind arguments to parameters
      std::list<std::string> params;
      std::optional<Env> matches =
          PatternMatch(operas[0]->GetFunctionValue().param, operas[1], globals,
                       &params, line_num);
      if (!matches) {
        std::cerr << "internal error in call_function, pattern match failed"
                  << std::endl;
        exit(-1);
      }
      // Create the new frame and push it on the stack
      auto* scope = new Scope(*matches, params);
      auto* frame = new Frame(operas[0]->GetFunctionValue().name, Stack(scope),
                              Stack(Action::MakeStatementAction(
                                  operas[0]->GetFunctionValue().body)));
      state->stack.Push(frame);
      break;
    }
    case ValKind::StructType: {
      const Value* arg = CopyVal(operas[1], line_num);
      const Value* sv = Value::MakeStructValue(operas[0], arg);
      Frame* frame = state->stack.Top();
      frame->todo.Push(Action::MakeValAction(sv));
      break;
    }
    case ValKind::AlternativeConstructorValue: {
      const Value* arg = CopyVal(operas[1], line_num);
      const Value* av = Value::MakeAlternativeValue(
          operas[0]->GetAlternativeConstructorValue().alt_name,
          operas[0]->GetAlternativeConstructorValue().choice_name,
          state->heap.AllocateValue(arg));
      Frame* frame = state->stack.Top();
      frame->todo.Push(Action::MakeValAction(av));
      break;
    }
    default:
      std::cerr << line_num << ": in call, expected a function, not ";
      PrintValue(operas[0], std::cerr);
      std::cerr << std::endl;
      exit(-1);
  }
}

void DeallocateScope(int line_num, Scope* scope) {
  for (const auto& l : scope->locals) {
    std::optional<Address> a = scope->values.Get(l);
    if (!a) {
      std::cerr << "internal error in DeallocateScope" << std::endl;
      exit(-1);
    }
    state->heap.Deallocate(*a);
  }
}

void DeallocateLocals(int line_num, Frame* frame) {
  for (auto scope : frame->scopes) {
    DeallocateScope(line_num, scope);
  }
}

void CreateTuple(Frame* frame, Action* act, const Expression* exp) {
  //    { { (v1,...,vn) :: C, E, F} :: S, H}
  // -> { { `(v1,...,vn) :: C, E, F} :: S, H}
  std::vector<TupleElement> elements;
  auto f = exp->GetTupleLiteral().fields.begin();

  for (auto i = act->results.begin(); i != act->results.end(); ++i, ++f) {
    Address a = state->heap.AllocateValue(*i);  // copy?
    elements.push_back({.name = f->name, .address = a});
  }
  const Value* tv = Value::MakeTupleValue(std::move(elements));
  frame->todo.Pop(1);
  frame->todo.Push(Action::MakeValAction(tv));
}

// Returns an updated environment that includes the bindings of
//    pattern variables to their matched values, if matching succeeds.
//
// The names of the pattern variables are added to the vars parameter.
// Returns nullopt if the value doesn't match the pattern.
auto PatternMatch(const Value* p, const Value* v, Env values,
                  std::list<std::string>* vars, int line_num)
    -> std::optional<Env> {
  switch (p->tag()) {
    case ValKind::BindingPlaceholderValue: {
      Address a = state->heap.AllocateValue(CopyVal(v, line_num));
      vars->push_back(p->GetBindingPlaceholderValue().name);
      values.Set(p->GetBindingPlaceholderValue().name, a);
      return values;
    }
    case ValKind::TupleValue:
      switch (v->tag()) {
        case ValKind::TupleValue: {
          if (p->GetTupleValue().elements.size() !=
              v->GetTupleValue().elements.size()) {
            std::cerr << "runtime error: arity mismatch in tuple pattern match"
                      << std::endl;
            exit(-1);
          }
          for (const TupleElement& element : p->GetTupleValue().elements) {
            auto a = FindTupleField(element.name, v);
            if (a == std::nullopt) {
              std::cerr << "runtime error: field " << element.name << "not in ";
              PrintValue(v, std::cerr);
              std::cerr << std::endl;
              exit(-1);
            }
            std::optional<Env> matches = PatternMatch(
                state->heap.Read(element.address, line_num),
                state->heap.Read(*a, line_num), values, vars, line_num);
            if (!matches) {
              return std::nullopt;
            }
            values = *matches;
          }  // for
          return values;
        }
        default:
          std::cerr
              << "internal error, expected a tuple value in pattern, not ";
          PrintValue(v, std::cerr);
          std::cerr << std::endl;
          exit(-1);
      }
    case ValKind::AlternativeValue:
      switch (v->tag()) {
        case ValKind::AlternativeValue: {
          if (p->GetAlternativeValue().choice_name !=
                  v->GetAlternativeValue().choice_name ||
              p->GetAlternativeValue().alt_name !=
                  v->GetAlternativeValue().alt_name) {
            return std::nullopt;
          }
          std::optional<Env> matches = PatternMatch(
              state->heap.Read(p->GetAlternativeValue().argument, line_num),
              state->heap.Read(v->GetAlternativeValue().argument, line_num),
              values, vars, line_num);
          if (!matches) {
            return std::nullopt;
          }
          return *matches;
        }
        default:
          std::cerr
              << "internal error, expected a choice alternative in pattern, "
                 "not ";
          PrintValue(v, std::cerr);
          std::cerr << std::endl;
          exit(-1);
      }
    case ValKind::FunctionType:
      switch (v->tag()) {
        case ValKind::FunctionType: {
          std::optional<Env> matches =
              PatternMatch(p->GetFunctionType().param,
                           v->GetFunctionType().param, values, vars, line_num);
          if (!matches) {
            return std::nullopt;
          }
          return PatternMatch(p->GetFunctionType().ret,
                              v->GetFunctionType().ret, *matches, vars,
                              line_num);
        }
        default:
          return std::nullopt;
      }
    default:
      if (ValueEqual(p, v, line_num)) {
        return values;
      } else {
        return std::nullopt;
      }
  }
}

void PatternAssignment(const Value* pat, const Value* val, int line_num) {
  switch (pat->tag()) {
    case ValKind::PointerValue:
      state->heap.Write(ValToPtr(pat, line_num), CopyVal(val, line_num),
                        line_num);
      break;
    case ValKind::TupleValue: {
      switch (val->tag()) {
        case ValKind::TupleValue: {
          if (pat->GetTupleValue().elements.size() !=
              val->GetTupleValue().elements.size()) {
            std::cerr << "runtime error: arity mismatch in tuple pattern match"
                      << std::endl;
            exit(-1);
          }
          for (const TupleElement& element : pat->GetTupleValue().elements) {
            auto a = FindTupleField(element.name, val);
            if (a == std::nullopt) {
              std::cerr << "runtime error: field " << element.name << "not in ";
              PrintValue(val, std::cerr);
              std::cerr << std::endl;
              exit(-1);
            }
            PatternAssignment(state->heap.Read(element.address, line_num),
                              state->heap.Read(*a, line_num), line_num);
          }
          break;
        }
        default:
          std::cerr
              << "internal error, expected a tuple value on right-hand-side, "
                 "not ";
          PrintValue(val, std::cerr);
          std::cerr << std::endl;
          exit(-1);
      }
      break;
    }
    case ValKind::AlternativeValue: {
      switch (val->tag()) {
        case ValKind::AlternativeValue: {
          if (pat->GetAlternativeValue().choice_name !=
                  val->GetAlternativeValue().choice_name ||
              pat->GetAlternativeValue().alt_name !=
                  val->GetAlternativeValue().alt_name) {
            std::cerr << "internal error in pattern assignment" << std::endl;
            exit(-1);
          }
          PatternAssignment(
              state->heap.Read(pat->GetAlternativeValue().argument, line_num),
              state->heap.Read(val->GetAlternativeValue().argument, line_num),
              line_num);
          break;
        }
        default:
          std::cerr
              << "internal error, expected an alternative in left-hand-side, "
                 "not ";
          PrintValue(val, std::cerr);
          std::cerr << std::endl;
          exit(-1);
      }
      break;
    }
    default:
      if (!ValueEqual(pat, val, line_num)) {
        std::cerr << "internal error in pattern assignment" << std::endl;
        exit(-1);
      }
  }
}

// State transitions for lvalues.

void StepLvalue() {
  Frame* frame = state->stack.Top();
  Action* act = frame->todo.Top();
  const Expression* exp = act->GetLValAction().exp;
  if (tracing_output) {
    std::cout << "--- step lvalue ";
    PrintExp(exp);
    std::cout << " --->" << std::endl;
  }
  switch (exp->tag()) {
    case ExpressionKind::IdentifierExpression: {
      //    { {x :: C, E, F} :: S, H}
      // -> { {E(x) :: C, E, F} :: S, H}
      std::optional<Address> pointer =
          CurrentEnv(state).Get(exp->GetIdentifierExpression().name);
      if (!pointer) {
        std::cerr << exp->line_num << ": could not find `"
                  << exp->GetIdentifierExpression().name << "`" << std::endl;
        exit(-1);
      }
      const Value* v = Value::MakePointerValue(*pointer);
      frame->todo.Pop();
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
    case ExpressionKind::FieldAccessExpression: {
      if (act->pos == 0) {
        //    { {e.f :: C, E, F} :: S, H}
        // -> { e :: [].f :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(
            exp->GetFieldAccessExpression().aggregate));
        act->pos++;
      } else {
        //    { v :: [].f :: C, E, F} :: S, H}
        // -> { { &v.f :: C, E, F} :: S, H }
        const Value* str = act->results[0];
        Address a = GetMember(str, exp->GetFieldAccessExpression().field,
                              exp->line_num);
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeValAction(Value::MakePointerValue(a)));
      }
      break;
    }
    case ExpressionKind::IndexExpression: {
      if (act->pos == 0) {
        //    { {e[i] :: C, E, F} :: S, H}
        // -> { e :: [][i] :: C, E, F} :: S, H}
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetIndexExpression().aggregate));
        act->pos++;
      } else if (act->pos == 1) {
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetIndexExpression().offset));
        act->pos++;
      } else if (act->pos == 2) {
        //    { v :: [][i] :: C, E, F} :: S, H}
        // -> { { &v[i] :: C, E, F} :: S, H }
        const Value* tuple = act->results[0];
        std::string f = std::to_string(ToInteger(act->results[1]));
        auto a = FindTupleField(f, tuple);
        if (a == std::nullopt) {
          std::cerr << "runtime error: field " << f << "not in ";
          PrintValue(tuple, std::cerr);
          std::cerr << std::endl;
          exit(-1);
        }
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeValAction(Value::MakePointerValue(*a)));
      }
      break;
    }
    case ExpressionKind::TupleLiteral: {
      if (act->pos == 0) {
        //    { {(f1=e1,...) :: C, E, F} :: S, H}
        // -> { {e1 :: (f1=[],...) :: C, E, F} :: S, H}
        const Expression* e1 = exp->GetTupleLiteral().fields[0].expression;
        frame->todo.Push(Action::MakeLValAction(e1));
        act->pos++;
      } else if (act->pos !=
                 static_cast<int>(exp->GetTupleLiteral().fields.size())) {
        //    { { vk :: (f1=v1,..., fk=[],fk+1=ek+1,...) :: C, E, F} :: S,
        //    H}
        // -> { { ek+1 :: (f1=v1,..., fk=vk, fk+1=[],...) :: C, E, F} :: S,
        // H}
        const Expression* elt =
            exp->GetTupleLiteral().fields[act->pos].expression;
        frame->todo.Push(Action::MakeLValAction(elt));
        act->pos++;
      } else {
        CreateTuple(frame, act, exp);
      }
      break;
    }
    case ExpressionKind::IntLiteral:
    case ExpressionKind::BoolLiteral:
    case ExpressionKind::CallExpression:
    case ExpressionKind::PrimitiveOperatorExpression:
    case ExpressionKind::IntTypeLiteral:
    case ExpressionKind::BoolTypeLiteral:
    case ExpressionKind::TypeTypeLiteral:
    case ExpressionKind::FunctionTypeLiteral:
    case ExpressionKind::AutoTypeLiteral:
    case ExpressionKind::ContinuationTypeLiteral:
    case ExpressionKind::BindingExpression: {
      std::cerr << "Can't treat expression as lvalue: ";
      PrintExp(exp);
      std::cerr << std::endl;
      exit(-1);
    }
  }
}

// State transitions for expressions.

void StepExp() {
  Frame* frame = state->stack.Top();
  Action* act = frame->todo.Top();
  const Expression* exp = act->GetExpressionAction().exp;
  if (tracing_output) {
    std::cout << "--- step exp ";
    PrintExp(exp);
    std::cout << " --->" << std::endl;
  }
  switch (exp->tag()) {
    case ExpressionKind::BindingExpression: {
      if (act->pos == 0) {
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetBindingExpression().type));
        act->pos++;
      } else {
        auto v = Value::MakeBindingPlaceholderValue(
            exp->GetBindingExpression().name, act->results[0]);
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeValAction(v));
      }
      break;
    }
    case ExpressionKind::IndexExpression: {
      if (act->pos == 0) {
        //    { { e[i] :: C, E, F} :: S, H}
        // -> { { e :: [][i] :: C, E, F} :: S, H}
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetIndexExpression().aggregate));
        act->pos++;
      } else if (act->pos == 1) {
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetIndexExpression().offset));
        act->pos++;
      } else if (act->pos == 2) {
        auto tuple = act->results[0];
        switch (tuple->tag()) {
          case ValKind::TupleValue: {
            //    { { v :: [][i] :: C, E, F} :: S, H}
            // -> { { v_i :: C, E, F} : S, H}
            std::string f = std::to_string(ToInteger(act->results[1]));
            auto a = FindTupleField(f, tuple);
            if (a == std::nullopt) {
              std::cerr << "runtime error, field " << f << " not in ";
              PrintValue(tuple, std::cerr);
              std::cerr << std::endl;
              exit(-1);
            }
            frame->todo.Pop(1);
            const Value* element = state->heap.Read(*a, exp->line_num);
            frame->todo.Push(Action::MakeValAction(element));
            break;
          }
          default:
            std::cerr
                << "runtime type error, expected a tuple in field access, "
                   "not ";
            PrintValue(tuple, std::cerr);
            exit(-1);
        }
      }
      break;
    }
    case ExpressionKind::TupleLiteral: {
      if (act->pos == 0) {
        if (exp->GetTupleLiteral().fields.size() > 0) {
          //    { {(f1=e1,...) :: C, E, F} :: S, H}
          // -> { {e1 :: (f1=[],...) :: C, E, F} :: S, H}
          const Expression* e1 = exp->GetTupleLiteral().fields[0].expression;
          frame->todo.Push(Action::MakeExpressionAction(e1));
          act->pos++;
        } else {
          CreateTuple(frame, act, exp);
        }
      } else if (act->pos !=
                 static_cast<int>(exp->GetTupleLiteral().fields.size())) {
        //    { { vk :: (f1=v1,..., fk=[],fk+1=ek+1,...) :: C, E, F} :: S,
        //    H}
        // -> { { ek+1 :: (f1=v1,..., fk=vk, fk+1=[],...) :: C, E, F} :: S,
        // H}
        const Expression* elt =
            exp->GetTupleLiteral().fields[act->pos].expression;
        frame->todo.Push(Action::MakeExpressionAction(elt));
        act->pos++;
      } else {
        CreateTuple(frame, act, exp);
      }
      break;
    }
    case ExpressionKind::FieldAccessExpression: {
      if (act->pos == 0) {
        //    { { e.f :: C, E, F} :: S, H}
        // -> { { e :: [].f :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(
            exp->GetFieldAccessExpression().aggregate));
        act->pos++;
      } else {
        //    { { v :: [].f :: C, E, F} :: S, H}
        // -> { { v_f :: C, E, F} : S, H}
        Address element =
            GetMember(act->results[0], exp->GetFieldAccessExpression().field,
                      exp->line_num);
        frame->todo.Pop(1);
        frame->todo.Push(
            Action::MakeValAction(state->heap.Read(element, exp->line_num)));
      }
      break;
    }
    case ExpressionKind::IdentifierExpression: {
      CHECK(act->pos == 0);
      // { {x :: C, E, F} :: S, H} -> { {H(E(x)) :: C, E, F} :: S, H}
      std::optional<Address> pointer =
          CurrentEnv(state).Get(exp->GetIdentifierExpression().name);
      if (!pointer) {
        std::cerr << exp->line_num << ": could not find `"
                  << exp->GetIdentifierExpression().name << "`" << std::endl;
        exit(-1);
      }
      const Value* pointee = state->heap.Read(*pointer, exp->line_num);
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(pointee));
      break;
    }
    case ExpressionKind::IntLiteral:
      CHECK(act->pos == 0);
      // { {n :: C, E, F} :: S, H} -> { {n' :: C, E, F} :: S, H}
      frame->todo.Pop(1);
      frame->todo.Push(
          Action::MakeValAction(Value::MakeIntValue(exp->GetIntLiteral())));
      break;
    case ExpressionKind::BoolLiteral:
      CHECK(act->pos == 0);
      // { {n :: C, E, F} :: S, H} -> { {n' :: C, E, F} :: S, H}
      frame->todo.Pop(1);
      frame->todo.Push(
          Action::MakeValAction(Value::MakeBoolValue(exp->GetBoolLiteral())));
      break;
    case ExpressionKind::PrimitiveOperatorExpression:
      if (act->pos !=
          static_cast<int>(
              exp->GetPrimitiveOperatorExpression().arguments.size())) {
        //    { {v :: op(vs,[],e,es) :: C, E, F} :: S, H}
        // -> { {e :: op(vs,v,[],es) :: C, E, F} :: S, H}
        const Expression* arg =
            exp->GetPrimitiveOperatorExpression().arguments[act->pos];
        frame->todo.Push(Action::MakeExpressionAction(arg));
        act->pos++;
      } else {
        //    { {v :: op(vs,[]) :: C, E, F} :: S, H}
        // -> { {eval_prim(op, (vs,v)) :: C, E, F} :: S, H}
        const Value* v = EvalPrim(exp->GetPrimitiveOperatorExpression().op,
                                  act->results, exp->line_num);
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeValAction(v));
      }
      break;
    case ExpressionKind::CallExpression:
      if (act->pos == 0) {
        //    { {e1(e2) :: C, E, F} :: S, H}
        // -> { {e1 :: [](e2) :: C, E, F} :: S, H}
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetCallExpression().function));
        act->pos++;
      } else if (act->pos == 1) {
        //    { { v :: [](e) :: C, E, F} :: S, H}
        // -> { { e :: v([]) :: C, E, F} :: S, H}
        frame->todo.Push(
            Action::MakeExpressionAction(exp->GetCallExpression().argument));
        act->pos++;
      } else if (act->pos == 2) {
        //    { { v2 :: v1([]) :: C, E, F} :: S, H}
        // -> { {C',E',F'} :: {C, E, F} :: S, H}
        frame->todo.Pop(1);
        CallFunction(exp->line_num, act->results, state);
      } else {
        std::cerr << "internal error in handle_value with Call" << std::endl;
        exit(-1);
      }
      break;
    case ExpressionKind::IntTypeLiteral: {
      CHECK(act->pos == 0);
      const Value* v = Value::MakeIntType();
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
    case ExpressionKind::BoolTypeLiteral: {
      CHECK(act->pos == 0);
      const Value* v = Value::MakeBoolType();
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
    case ExpressionKind::AutoTypeLiteral: {
      CHECK(act->pos == 0);
      const Value* v = Value::MakeAutoType();
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
    case ExpressionKind::TypeTypeLiteral: {
      CHECK(act->pos == 0);
      const Value* v = Value::MakeTypeType();
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
    case ExpressionKind::FunctionTypeLiteral: {
      if (act->pos == 0) {
        frame->todo.Push(Action::MakeExpressionAction(
            exp->GetFunctionTypeLiteral().parameter));
        act->pos++;
      } else if (act->pos == 1) {
        //    { { pt :: fn [] -> e :: C, E, F} :: S, H}
        // -> { { e :: fn pt -> []) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(
            exp->GetFunctionTypeLiteral().return_type));
        act->pos++;
      } else if (act->pos == 2) {
        //    { { rt :: fn pt -> [] :: C, E, F} :: S, H}
        // -> { fn pt -> rt :: {C, E, F} :: S, H}
        const Value* v =
            Value::MakeFunctionType(act->results[0], act->results[1]);
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeValAction(v));
      }
      break;
    }
    case ExpressionKind::ContinuationTypeLiteral: {
      CHECK(act->pos == 0);
      const Value* v = Value::MakeContinuationType();
      frame->todo.Pop(1);
      frame->todo.Push(Action::MakeValAction(v));
      break;
    }
  }  // switch (exp->tag)
}

auto IsWhileAct(Action* act) -> bool {
  switch (act->tag()) {
    case ActionKind::StatementAction:
      switch (act->GetStatementAction().stmt->tag) {
        case StatementKind::While:
          return true;
        default:
          return false;
      }
    default:
      return false;
  }
}

auto IsBlockAct(Action* act) -> bool {
  switch (act->tag()) {
    case ActionKind::StatementAction:
      switch (act->GetStatementAction().stmt->tag) {
        case StatementKind::Block:
          return true;
        default:
          return false;
      }
    default:
      return false;
  }
}

// State transitions for statements.

void StepStmt() {
  Frame* frame = state->stack.Top();
  Action* act = frame->todo.Top();
  const Statement* stmt = act->GetStatementAction().stmt;
  CHECK(stmt != nullptr && "null statement!");
  if (tracing_output) {
    std::cout << "--- step stmt ";
    PrintStatement(stmt, 1);
    std::cout << " --->" << std::endl;
  }
  switch (stmt->tag) {
    case StatementKind::Match:
      if (act->pos == 0) {
        //    { { (match (e) ...) :: C, E, F} :: S, H}
        // -> { { e :: (match ([]) ...) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetMatch().exp));
        act->pos++;
      } else {
        // Regarding act->pos:
        // * odd: start interpreting the pattern of a clause
        // * even: finished interpreting the pattern, now try to match
        //
        // Regarding act->results:
        // * 0: the value that we're matching
        // * 1: the pattern for clause 0
        // * 2: the pattern for clause 1
        // * ...
        auto clause_num = (act->pos - 1) / 2;
        if (clause_num >= static_cast<int>(stmt->GetMatch().clauses->size())) {
          frame->todo.Pop(1);
          break;
        }
        auto c = stmt->GetMatch().clauses->begin();
        std::advance(c, clause_num);

        if (act->pos % 2 == 1) {
          // start interpreting the pattern of the clause
          //    { {v :: (match ([]) ...) :: C, E, F} :: S, H}
          // -> { {pi :: (match ([]) ...) :: C, E, F} :: S, H}
          frame->todo.Push(Action::MakeExpressionAction(c->first));
          act->pos++;
        } else {  // try to match
          auto v = act->results[0];
          auto pat = act->results[clause_num + 1];
          auto values = CurrentEnv(state);
          std::list<std::string> vars;
          std::optional<Env> matches =
              PatternMatch(pat, v, values, &vars, stmt->line_num);
          if (matches) {  // we have a match, start the body
            auto* new_scope = new Scope(*matches, vars);
            frame->scopes.Push(new_scope);
            const Statement* body_block =
                Statement::MakeBlock(stmt->line_num, c->second);
            Action* body_act = Action::MakeStatementAction(body_block);
            body_act->pos = 1;
            frame->todo.Pop(1);
            frame->todo.Push(body_act);
            frame->todo.Push(Action::MakeStatementAction(c->second));
          } else {
            // this case did not match, moving on
            act->pos++;
            clause_num = (act->pos - 1) / 2;
            if (clause_num ==
                static_cast<int>(stmt->GetMatch().clauses->size())) {
              frame->todo.Pop(2);
            }
          }
        }
      }
      break;
    case StatementKind::While:
      if (act->pos == 0) {
        //    { { (while (e) s) :: C, E, F} :: S, H}
        // -> { { e :: (while ([]) s) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetWhile().cond));
        act->pos++;
      } else if (ValToBool(act->results[0], stmt->line_num)) {
        //    { {true :: (while ([]) s) :: C, E, F} :: S, H}
        // -> { { s :: (while (e) s) :: C, E, F } :: S, H}
        frame->todo.Top()->pos = 0;
        frame->todo.Top()->results.clear();
        frame->todo.Push(Action::MakeStatementAction(stmt->GetWhile().body));
      } else {
        //    { {false :: (while ([]) s) :: C, E, F} :: S, H}
        // -> { { C, E, F } :: S, H}
        frame->todo.Top()->pos = 0;
        frame->todo.Top()->results.clear();
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::Break:
      CHECK(act->pos == 0);
      //    { { break; :: ... :: (while (e) s) :: C, E, F} :: S, H}
      // -> { { C, E', F} :: S, H}
      frame->todo.Pop(1);
      while (!frame->todo.IsEmpty() && !IsWhileAct(frame->todo.Top())) {
        if (IsBlockAct(frame->todo.Top())) {
          DeallocateScope(stmt->line_num, frame->scopes.Top());
          frame->scopes.Pop(1);
        }
        frame->todo.Pop(1);
      }
      frame->todo.Pop(1);
      break;
    case StatementKind::Continue:
      CHECK(act->pos == 0);
      //    { { continue; :: ... :: (while (e) s) :: C, E, F} :: S, H}
      // -> { { (while (e) s) :: C, E', F} :: S, H}
      frame->todo.Pop(1);
      while (!frame->todo.IsEmpty() && !IsWhileAct(frame->todo.Top())) {
        if (IsBlockAct(frame->todo.Top())) {
          DeallocateScope(stmt->line_num, frame->scopes.Top());
          frame->scopes.Pop(1);
        }
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::Block: {
      if (act->pos == 0) {
        if (stmt->GetBlock().stmt) {
          auto* scope = new Scope(CurrentEnv(state), {});
          frame->scopes.Push(scope);
          frame->todo.Push(Action::MakeStatementAction(stmt->GetBlock().stmt));
          act->pos++;
          act->pos++;
        } else {
          frame->todo.Pop();
        }
      } else {
        Scope* scope = frame->scopes.Top();
        DeallocateScope(stmt->line_num, scope);
        frame->scopes.Pop(1);
        frame->todo.Pop(1);
      }
      break;
    }
    case StatementKind::VariableDefinition:
      if (act->pos == 0) {
        //    { {(var x = e) :: C, E, F} :: S, H}
        // -> { {e :: (var x = []) :: C, E, F} :: S, H}
        frame->todo.Push(
            Action::MakeExpressionAction(stmt->GetVariableDefinition().init));
        act->pos++;
      } else if (act->pos == 1) {
        frame->todo.Push(
            Action::MakeExpressionAction(stmt->GetVariableDefinition().pat));
        act->pos++;
      } else if (act->pos == 2) {
        //    { { v :: (x = []) :: C, E, F} :: S, H}
        // -> { { C, E(x := a), F} :: S, H(a := copy(v))}
        const Value* v = act->results[0];
        const Value* p = act->results[1];

        std::optional<Env> matches =
            PatternMatch(p, v, frame->scopes.Top()->values,
                         &frame->scopes.Top()->locals, stmt->line_num);
        if (!matches) {
          std::cerr << stmt->line_num
                    << ": internal error in variable definition, match failed"
                    << std::endl;
          exit(-1);
        }
        frame->scopes.Top()->values = *matches;
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::ExpressionStatement:
      if (act->pos == 0) {
        //    { {e :: C, E, F} :: S, H}
        // -> { {e :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetExpression()));
        act->pos++;
      } else {
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::Assign:
      if (act->pos == 0) {
        //    { {(lv = e) :: C, E, F} :: S, H}
        // -> { {lv :: ([] = e) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeLValAction(stmt->GetAssign().lhs));
        act->pos++;
      } else if (act->pos == 1) {
        //    { { a :: ([] = e) :: C, E, F} :: S, H}
        // -> { { e :: (a = []) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetAssign().rhs));
        act->pos++;
      } else if (act->pos == 2) {
        //    { { v :: (a = []) :: C, E, F} :: S, H}
        // -> { { C, E, F} :: S, H(a := v)}
        auto pat = act->results[0];
        auto val = act->results[1];
        PatternAssignment(pat, val, stmt->line_num);
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::If:
      if (act->pos == 0) {
        //    { {(if (e) then_stmt else else_stmt) :: C, E, F} :: S, H}
        // -> { { e :: (if ([]) then_stmt else else_stmt) :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetIf().cond));
        act->pos++;
      } else if (ValToBool(act->results[0], stmt->line_num)) {
        //    { {true :: if ([]) then_stmt else else_stmt :: C, E, F} ::
        //      S, H}
        // -> { { then_stmt :: C, E, F } :: S, H}
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeStatementAction(stmt->GetIf().then_stmt));
      } else if (stmt->GetIf().else_stmt) {
        //    { {false :: if ([]) then_stmt else else_stmt :: C, E, F} ::
        //      S, H}
        // -> { { else_stmt :: C, E, F } :: S, H}
        frame->todo.Pop(1);
        frame->todo.Push(Action::MakeStatementAction(stmt->GetIf().else_stmt));
      } else {
        frame->todo.Pop(1);
      }
      break;
    case StatementKind::Return:
      if (act->pos == 0) {
        //    { {return e :: C, E, F} :: S, H}
        // -> { {e :: return [] :: C, E, F} :: S, H}
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetReturn()));
        act->pos++;
      } else {
        //    { {v :: return [] :: C, E, F} :: {C', E', F'} :: S, H}
        // -> { {v :: C', E', F'} :: S, H}
        const Value* ret_val = CopyVal(act->results[0], stmt->line_num);
        DeallocateLocals(stmt->line_num, frame);
        state->stack.Pop(1);
        frame = state->stack.Top();
        frame->todo.Push(Action::MakeValAction(ret_val));
      }
      break;
    case StatementKind::Sequence:
      CHECK(act->pos == 0);
      //    { { (s1,s2) :: C, E, F} :: S, H}
      // -> { { s1 :: s2 :: C, E, F} :: S, H}
      frame->todo.Pop(1);
      if (stmt->GetSequence().next) {
        frame->todo.Push(Action::MakeStatementAction(stmt->GetSequence().next));
      }
      frame->todo.Push(Action::MakeStatementAction(stmt->GetSequence().stmt));
      break;
    case StatementKind::Continuation: {
      CHECK(act->pos == 0);
      // Create a continuation object by creating a frame similar the
      // way one is created in a function call.
      Scope* scope = new Scope(CurrentEnv(state), std::list<std::string>());
      Stack<Scope*> scopes;
      scopes.Push(scope);
      Stack<Action*> todo;
      todo.Push(Action::MakeStatementAction(Statement::MakeReturn(
          stmt->line_num, Expression::MakeTupleLiteral(stmt->line_num, {}))));
      todo.Push(Action::MakeStatementAction(stmt->GetContinuation().body));
      Frame* continuation_frame = new Frame("__continuation", scopes, todo);
      Address continuation_address = state->heap.AllocateValue(
          Value::MakeContinuationValue({continuation_frame}));
      // Store the continuation's address in the frame.
      continuation_frame->continuation = continuation_address;
      // Bind the continuation object to the continuation variable
      frame->scopes.Top()->values.Set(
          *stmt->GetContinuation().continuation_variable, continuation_address);
      // Pop the continuation statement.
      frame->todo.Pop();
      break;
    }
    case StatementKind::Run:
      if (act->pos == 0) {
        // Evaluate the argument of the run statement.
        frame->todo.Push(Action::MakeExpressionAction(stmt->GetRun().argument));
        act->pos++;
      } else {
        frame->todo.Pop(1);
        // Push an expression statement action to ignore the result
        // value from the continuation.
        Action* ignore_result = Action::MakeStatementAction(
            Statement::MakeExpStmt(stmt->line_num, Expression::MakeTupleLiteral(
                                                       stmt->line_num, {})));
        ignore_result->pos = 0;
        frame->todo.Push(ignore_result);
        // Push the continuation onto the current stack.
        std::vector<Frame*> continuation_vector =
            ContinuationToVector(act->results[0], stmt->line_num);
        for (auto frame_iter = continuation_vector.rbegin();
             frame_iter != continuation_vector.rend(); ++frame_iter) {
          state->stack.Push(*frame_iter);
        }
      }
      break;
    case StatementKind::Await:
      CHECK(act->pos == 0);
      // Pause the current continuation
      frame->todo.Pop();
      std::vector<Frame*> paused;
      do {
        paused.push_back(state->stack.Pop());
      } while (!paused.back()->IsContinuation());
      // Update the continuation with the paused stack.
      state->heap.Write(paused.back()->continuation,
                        Value::MakeContinuationValue(paused), stmt->line_num);
      break;
  }
}

auto GetMember(const Value* v, const std::string& f, int line_num) -> Address {
  switch (v->tag()) {
    case ValKind::StructValue: {
      auto a = FindTupleField(f, v->GetStructValue().inits);
      if (a == std::nullopt) {
        std::cerr << "runtime error, member " << f << " not in ";
        PrintValue(v, std::cerr);
        std::cerr << std::endl;
        exit(-1);
      }
      return *a;
    }
    case ValKind::TupleValue: {
      auto a = FindTupleField(f, v);
      if (a == std::nullopt) {
        std::cerr << "field " << f << " not in ";
        PrintValue(v, std::cerr);
        std::cerr << std::endl;
        exit(-1);
      }
      return *a;
    }
    case ValKind::ChoiceType: {
      if (FindInVarValues(f, v->GetChoiceType().alternatives) == nullptr) {
        std::cerr << "alternative " << f << " not in ";
        PrintValue(v, std::cerr);
        std::cerr << std::endl;
        exit(-1);
      }
      auto ac =
          Value::MakeAlternativeConstructorValue(f, v->GetChoiceType().name);
      return state->heap.AllocateValue(ac);
    }
    default:
      std::cerr << "field access not allowed for value ";
      PrintValue(v, std::cerr);
      std::cerr << std::endl;
      exit(-1);
  }
}

// State transition.
void Step() {
  Frame* frame = state->stack.Top();
  if (frame->todo.IsEmpty()) {
    std::cerr << "runtime error: fell off end of function " << frame->name
              << " without `return`" << std::endl;
    exit(-1);
  }

  Action* act = frame->todo.Top();
  switch (act->tag()) {
    case ActionKind::ValAction: {
      Action* val_act = frame->todo.Pop();
      Action* act = frame->todo.Top();
      act->results.push_back(val_act->GetValAction().val);
      break;
    }
    case ActionKind::LValAction:
      StepLvalue();
      break;
    case ActionKind::ExpressionAction:
      StepExp();
      break;
    case ActionKind::StatementAction:
      StepStmt();
      break;
  }  // switch
}

// Interpret the whole porogram.
auto InterpProgram(std::list<Declaration>* fs) -> int {
  state = new State();  // Runtime state.
  if (tracing_output) {
    std::cout << "********** initializing globals **********" << std::endl;
  }
  InitGlobals(fs);

  const Expression* arg = Expression::MakeTupleLiteral(0, {});
  const Expression* call_main = Expression::MakeCallExpression(
      0, Expression::MakeIdentifierExpression(0, "main"), arg);
  auto todo = Stack(Action::MakeExpressionAction(call_main));
  auto* scope = new Scope(globals, std::list<std::string>());
  auto* frame = new Frame("top", Stack(scope), todo);
  state->stack = Stack(frame);

  if (tracing_output) {
    std::cout << "********** calling main function **********" << std::endl;
    PrintState(std::cout);
  }

  while (state->stack.CountExceeds(1) ||
         state->stack.Top()->todo.CountExceeds(1) ||
         state->stack.Top()->todo.Top()->tag() != ActionKind::ValAction) {
    Step();
    if (tracing_output) {
      PrintState(std::cout);
    }
  }
  const Value* v = state->stack.Top()->todo.Top()->GetValAction().val;
  return ValToInt(v, 0);
}

// Interpret an expression at compile-time.
auto InterpExp(Env values, const Expression* e) -> const Value* {
  auto todo = Stack(Action::MakeExpressionAction(e));
  auto* scope = new Scope(values, std::list<std::string>());
  auto* frame = new Frame("InterpExp", Stack(scope), todo);
  state->stack = Stack(frame);

  while (state->stack.CountExceeds(1) ||
         state->stack.Top()->todo.CountExceeds(1) ||
         state->stack.Top()->todo.Top()->tag() != ActionKind::ValAction) {
    Step();
  }
  const Value* v = state->stack.Top()->todo.Top()->GetValAction().val;
  return v;
}

}  // namespace Carbon